1. Field of the Invention
The invention relates to a premixing burner for generating an ignitable fuel/air mixture, with a swirl generator which provides at least two burner shells which complement one another to form a throughflow body and which jointly enclose an axially conically widening swirl space and delimit with respect to one another, in the axial longitudinal extent of the cone, tangential air inlet slits, through which combustion supply air passes into the swirl space in which an axially propagating swirl flow is formed, and with means for the injection of fuel, with are provided at least partially along the tangentially running air inlet slits.
2. Brief Description of the Related Art
Premixing burners of the abovementioned generic type are known from a multiplicity of previous publications, such as, for example, EP 0 210 462 A1 and EP 0 321 809 B1, to name only a few. Premixing burners of this type are based on the general active principle of generating, within a mostly conically designed swirl generator providing at least two part conical shells assembled with a correspondingly mutual overlap, a swirl flow of a fuel/air mixture and which is ignited within a combustion chamber following the premixing burner in the flow direction, so as to form a premixing flame which is spatially as stable as possible.
For swirl generation, the part conical shells overlapping with one another enclose, along the burner axis, tangential air inlet slits, through which air passes radially into the swirl space delimited by the part conical shells, so as to impart a swirl flow propagating along the burner axis. The part conical shells, mostly with double-walled design, provide for the supply of fuel, in the region along the air inlet slits, at least one internal fuel supply duct, through which is supplied in each case gaseous fuel which emerges via fuel nozzle orifices into the region of the air inlet slits. For this purpose, the fuel orifices are provided, distributed, in the region of the burner shell wall facing the air inlet slit, in order thereby, even in the region of the air inlet slit, to ensure effective intermixing, as uniform as possible, between the gaseous fuel and the inflowing supply air.
In addition to the double-walled design of the part conical shells delimiting the swirl space, it is also known to use part conical shells which are themselves formed simply from single-walled flat materials for air deflection. Premixing burners of this type provide in each case, along the onflow edge of the part conical shells, an attachment in the form of a pipeline, through which gaseous fuel is fed into the combustion supply air along the tangential extent of the air inlet slit through bores provided correspondingly in the pipeline. For this purpose, the pipeline is connected fixedly to the onflow edge of the part conical shell in the manner of a soldered or welded joint.
For reasons of operating reliability which must always be ensured, the supply of gaseous fuel for further feed along the fuel orifices into the area of the air inlet slits normally takes place at gas temperatures in range of between 20° C. and 30° C. On the other hand, as a consequence of operation, temperatures of between 300° C. and 350° C. prevail on account of the radiation temperatures prevailing in the region of the air inlet slits. It is clear that all those part conical shell surfaces delimiting the air inlet regions have body temperatures in the range of the above radiation temperatures. On the other hand, the part conical shell regions are cooled directly around the fuel orifices by the cool gas stream. Owing to these temperature differences, high thermal gradients occur in the region of the fuel orifices and lead to cracks within the material regions surrounding the fuel orifices. This results in irreversible structural weakenings which may possibly lead to a total loss of at least the affected part conical shell. Moreover, the risk of local flashbacks into the duct regions of the fuel supply increases in cracked fuel orifices, and, as a consequence, even the operating reliability of a premixing burner weakened in this way is ultimately called into question.